The present invention generally relates to a signal processing circuit of a video camera, and particularly to a signal processing circuit suitable for the improvement in quality at a low luminance portion in a video camera employing a single image pickup tube in order to obtain a color video signal.
It is important to make a home color video camera small and light and a single-tube color camera of the frequency separation type in which a single image pickup tube is provided with two sets of color stripe filters is widely used. As to such a single-tube color camera, some systems are disclosed, for example, in "THE JOURNAL OF THE INSTITUTE OF TELEVISION ENGINEERS OF JAPAN" March, 1981, vol. 35, No. 3, pp. 183-187. In brief, a luminance signal is derived as a base band signal from a target of an image pickup tube. A color stripe filter is provided with, for example, a red cut filter and a blue cut filter which are arranged to slant in the opposite directions to each other on a photoconductive surface of the image pickup tube, and a chrominance signal is derived as a multiple frequency carrier chrominance signal having frequency components determined depending on the arrangement of the color stripe filter. The luminance signal and the chrominance signal are amplified in the mixed state by a preamplifier and the black level is clamped by a clamp circuit. The signal DC restored by the clamp circuit is processed to be a predetermined video signal by a signal processing circuit which performs blanking processing, automatic gain control, and so on. The output of the signal processing circuit is passed through a low pass filter having a cut off frequency of 3 MHz to obtain a luminance signal Y. The output of the signal processing circuit is passed also through a low pass filter having a cut off frequency of 500 KHz to obtain a low frequency band component Y.sub.L of the luminance signal for obtaining a color difference signal. Further, for example, a carrier chrominance signal having quadrature modulated red and blue components can be obtained from the output of the signal processing circuit through a band pass filter, for example, having a band width of .+-.0.5 MHz with a center frequency of 3.58 MHz. The thus obtained quadrature modulated carrier chrominance signal is separated on the basis of the principle of vertical correlation of a picture image into a red carrier chrominance signal and a blue carrier chrominance signal through addition/subtraction operations as to a one-horizontal-period delayed line and the input and output of this delayed line. The two separated signals are amplitude-detected to obtain a red signal (R) and a blue signal (B), respectively. These signals Y, Y.sub.L, R, and B are converted into a predetermined color video signal through a known encoder.
In such a single-tube color video camera, there are several points to be improved in picture quality at a low luminance portion.
In the case of an image pickup tube, generally, there occurs a residual image due to residual charges caused by the fact that charges can not be charged/discharged by one scanning because of the time constant determined by the scanning electron beam impedance and the storage capacitance of the photoconductive film. This phenomenon is remarkable at low luminance, and in the case of a single carrier frequency separation system, this residual image may be a magenta/green colored residual which will deteriorate picture quality. In order to reduce such a residual image, a method is practically used in which a bias light is additionally applied from the frontal or rear side of the photoconductive film, for example, by an LED so as to increase the potential at the rear side of the target to thereby lower the beam impedance during a dark period, even if there is no incident light on the camera lens. However, it is difficult to uniformly irradiate the photoconductive film by a bias light, and shading occurs at a dark portion. A shading may occur at a dark portion by nonuniformity in sensitivity due to deflection, or the like, resulting in deterioration in picture quality. Such a shading has a parabolic component and a saw-tooth component. In order to correct such a shading, means is known, as shown in FIG. 8 at page 186 of the above-mentioned reference, in which an optical black portion correction pulse is added to a video signal obtained from an image pickup tube, and the pulse is modulated with a vertical saw-tooth waveform and a parabolic waveform and added to the video signal and clamped.
In this shading correction circuit, however, a correction pulse oscillator circuit, a pulse modulator circuit, and so on, are additionally required.
Further, at a dark portion of a picture image, the color phase (difference in phase between the chrominance primary signal and the chrominance carrier reference) and the color noise may be significant and the reduction in S/N ratio due to increase in amplifier gain by the automatic gain control may increase color noise. In this regard, it has been suggested to provide a base clip circuit in the path of a carrier chrominance signal obtained through an encoder to reduce such color phase and color noise. The base clip circuit has an input/output characteristic which is non-linear at a small level to suppress a portion of a carrier chrominance signal having a level lower than a threshold level. If the level of the chrominance signal is small, the level becomes small not only at a dark portion, but also at a non-colored portion of a bright portion.
If the threshold of the base clip circuit is determined so as to make the color phase and the color noise not significant at a bright portion, it becomes difficult to sufficiently obtain an improvement in color noise at a dark portion or when a dark object is photographed. On the other hand, if the threshold of the base clip circuit is determined to be sufficiently large that the color phase and the color noise causes no problem even at a dark portion or when a dark object is photographed, the color signal information of a colored picture is also greatly suppressed so that color saturation at a bright portion is lost, thereby making it impossible to obtain a proper picture image.